一次夜间β中尺度弓形回波形成机制机理研究  

Study on the Formation Mechanism of a NocturnalMeso-β Scale Bow Echo

作　　者：孙敏[1,2,3] 赵畅 吴君婧 孔晓宇 SUN Min;ZHAO Chang;WU Junjing;KONG Xiaoyu(Shanghai Central Meteorological Observatory,Shanghai 200030;Key Laboratory of Mesoscale Severe Weather/Ministry of Education and School of Atmospheric Sciences,Nanjing University,Nanjing 210023;Joint Laboratory of Phased Array Weather Radar,Shanghai 200030)

机构地区：[1]上海中心气象台,上海200030 [2]南京大学大气科学学院中尺度灾害性天气教育部重点实验室,南京210023 [3]相控阵阵列天气雷达联合实验室,上海200030

出　　处：《气象》2023年第10期1187-1202,共16页Meteorological Monthly

基　　金：国家自然科学基金项目(41775049);中国气象局创新发展专项(CXFZ2022J004);上海台风研究基金项目(TFJJ202014);“上海市气象局强对流科技创新团队”共同资助。

摘　　要：2017年9月24日夜间至25日凌晨,沿长江一线发生了一次夜间强对流过程。准东西向锋面雨带南侧垂直于锋面走向的β中尺度对流系统由线状逐渐演变为弓形,造成长江中下游地区产生短时强降水,并伴随7级雷暴大风。从环境场来看,夜间不存在有利的热力条件,预报难度较大。本研究利用观测和数值模拟对弓形回波的形成和演变机制机理进行分析,雷达观测显示初始阶段有一条东北—西南向的β中尺度线状对流带,在其西南侧不断有新生的对流单体合并进入对流主体,形成侧后向传播,之后在对流主体移动方向前侧(东南侧)又有新的对流单体生成,逐渐发展成西北—东南向的带状,并向东北方向移动,最终导致原β中尺度线状对流带演变并加强为弓形回波。高分辨率数值模式模拟的对流系统演变过程与实况十分接近,利用涡度方程进行诊断分析显示涡度的倾侧项在侧后向传播中发挥了重要的作用。对流发展初期,在倾侧项作用下回波西南侧有新的对流单体生成并与主体回波合并,随着回波不断合并增强,辐散项的作用逐渐增大,主体回波在西南侧的倾侧项和东北侧的辐散项共同作用下正涡度明显增大,且其垂直平流项将正涡度向上传播,有利于对流的垂直伸展。在主体回波前侧,受水平平流项的作用不断有新的对流单体生成,但由于垂直伸展高度低,受低层风引导向东北方向移动,在移动过程中对流单体排列呈西北—东南向且逐渐合并涡度增大,最终导致线状主体回波演变为弓形回波。此次弓形回波的形成过程与经典模型存在显著差异,其弓形后侧没有明显的后侧入流急流,而是具有明显的前侧近地层入流,主要受到了前侧暖区内对流系统发展影响。This study is concerned with a nocturnal convective process from the night of the 24th to the early morning of the 25th September 2017.The meso-βscale convective system on the south side of the frontal rainband perpendicular to the quasi-east-west frontal orientation gradually evolved into a bow echo,causing short-time severe precipitation in the middle and lower reaches of the Yangtze River,accompanied by Level 7 thunderstorm gale.From the perspective of the large-scale environment,there is no favorable thermodynamic conditions at night.Therefore it is difficult to make a forecast.In this study observations and numerical simulations are used to analyze the fomation mechanism.The radar observations show that there is a northeast-southwest meso-βscale convective belt,moving in the south-easterly direction,and there are new convective cells being triggered on its southwest side,forming the lateral back-building propagation.The new cells then merge into the main convective zone.Other new cells,generated ahead(southeast side)of the main convective zone,gradually develop into a northwest-southeast belt and move to the northeast,eventually making the original main northeast-southwest convective belt gradually strengthen and finally evolve into a bow echo.Although the high-resolution numerical model simulation results deviate from observations in intensity and time,the convective system evolution processes are very close to the observations.Therefore,the vertical vorticity equation is used to diagnose the mechanism.The results show that the vorticity tilt term plays an important role in the lateral back-building propagation.In the early stage of the convection development,new cells are generated on the southwest side under the effect of the vorticity tilt term and merge with the main echo.As the echoes continuously merge and strengthen,the divergence term becomes more important and the positive vorticity of the main echo increases significantly under both the vorticity tilt term and the divergence term.Besides,the vor

关 键 词：侧后向传播 弓形回波 涡度倾侧项 涡度水平平流项 

分 类 号：P458[天文地球—大气科学及气象学]